1. Field of the Invention
This application relates to video encoding, more specifically, to techniques for interpolating reference frames in video compression systems.
2. Description of the Related Art
Digital video capabilities can be incorporated into a wide range of devices, including digital televisions, digital direct broadcast systems, wireless communication devices, personal digital assistants (PDAs), laptop computers, desktop computers, digital cameras, digital recording devices, cellular or satellite radio telephones, and the like. These and other digital video devices can provide significant improvements over conventional analog video systems in creating, modifying, transmitting, storing, recording and playing full motion video sequences.
A number of different video encoding standards have been established for communicating digital video sequences. The Moving Picture Experts Group (MPEG), for example, has developed a number of standards including MPEG-1, MPEG-2 and MPEG-4. Other standards include ITU H.263, QuickTime™ technology developed by Apple Computer of Cupertino Calif., Video for Windows™ developed by Microsoft Corporation of Redmond, Wash., Indeo™ developed by Intel Corporation, RealVideo™ from RealNetworks, Inc. of Seattle, Wash., and Cinepak™ developed by SuperMac, Inc. These and other standards, including standards yet to be developed, will continue to evolve.
Video encoding standards achieve increased transmission rates by encoding data in a compressed fashion. Compression can reduce the overall amount of data that needs to be transmitted for effective transmission of image frames. The MPEG standards, for example, utilize graphics and video compression techniques designed to facilitate video and image transmission over a narrower bandwidth than could be achieved without the compression. In particular, the MPEG standards incorporate video encoding techniques that utilize similarities between successive image frames, referred to as temporal or inter-frame correlation, to provide inter-frame compression. The inter-frame compression techniques exploit data redundancy across frames by using motion compensated prediction, i.e. by predicting a frame from another after estimating the motion of the scene. In addition, the video encoding techniques may utilize similarities within image frames, referred to as spatial or intra-frame correlation. Frame compression is typically based upon conventional processes for compressing still images, such as discrete cosine transform (DCT) encoding. Discrete cosine transform (DCT) encoding is also used to compress the motion compensated prediction.
One DCT technique is known as the Adaptive Block Size Discrete Cosine Transform (ABSDCT) method. This technique is disclosed in U.S. Pat. No. 5,021,891, entitled “Adaptive Block Size Image Compression Method And System,” assigned to the assignee of the present invention and incorporated herein by reference. DCT techniques are also disclosed in U.S. Pat. No. 5,107,345, entitled “Adaptive Block Size Image Compression Method And System,” assigned to the assignee of the present invention and incorporated herein by reference. Further, the use of the ABSDCT technique in combination with a Differential Quadtree Transform technique is discussed in U.S. Pat. No. 5,452,104, entitled “Adaptive Block Size Image Compression Method And System,” also assigned to the assignee of the present invention and incorporated herein by reference. The systems disclosed in these patents utilize what is referred to as “intra-frame” encoding, where each frame of image, data is encoded without regard to the content of any other frame. Using the ABSDCT technique, the achievable data rate may be reduced from around 1.5 billion bits per second to approximately 50 million bits per second without discernible degradation of the image quality.
The ABSDCT technique may be used to compress either a black and white or a color image or signal representing the image. The color input signal may be in a YIQ format, with Y being the luminance, or brightness, sample, and I and Q being the chrominance, or color, samples for each 4.times.4 block of pixels. Other known formats such as the YUV, YC.sub.bC.sub.y or RGB formats may also be used. Because of the low spatial sensitivity of the eye to color, most research has shown that a sub-sample of the color components by a factor of four in the horizontal and vertical directions is reasonable. Accordingly, a video signal may be represented by four luminance components and two chrominance components.
To support the compression techniques, many digital video devices include an encoder for compressing digital video sequences, and a decoder for decompressing the digital video sequences. In many cases, the encoder and decoder comprise an integrated encoder/decoder (CODEC) that operates on blocks of pixels within frames that define the sequence of video images. In the MPEG-4 standard, for example, the encoder of a sending device typically divides a video image frame to be transmitted into macroblocks comprising smaller image blocks. For each macroblock in the image frame, the encoder searches macroblocks of the immediately preceding video frame to identify the most similar macroblock, and encodes the difference between the macroblocks for transmission, along with a motion vector that indicates which macroblock from the previous frame was used for encoding. The decoder of a receiving device receives the motion vector and encoded differences, and performs motion compensation to generate video sequences. Motion vectors may have full, half or quarter pixel precisions, depending on the level of precision selected by an encoder. When motion vectors with fractional pixel values are used, a better prediction block is obtained. Interpolation is carried out to determine the values of the fractional pixels (sub-pixels). In one example, pixel values include bits representing the intensity of a luminance, chrominance, or color component.
The video encoding process is computationally intensive. In particular, the process of comparing video blocks to previously transmitted video blocks requires large numbers of computations. Improved encoding techniques are highly desirable, particularly for use in wireless devices or other portable video devices where computational resources are more limited and power consumption is a concern.